The present invention relates to a charge pump circuit, and more particularly, to a charge pump circuit suitable for feedback systems such as phase locked loop (PLL) circuits and delay locked loop (DLL) circuits.
In general, in feedback systems such as PLL circuits and DLL circuits, a charge pump circuit is used for generating a signal for controlling a voltage-controlled oscillator (VCO) and a voltage-controlled delay circuit (VCD). FIG. 7 shows a configuration of a conventional charge pump circuit. When a signal UP is activated, a switch 101 is turned ON allowing a current to be supplied from a current source 102, to thereby implement “push” operation. When a signal DN is activated, a switch 103 is turned ON allowing a current to be drawn to a current source 104, to thereby implement “pull” operation. The current related to such “push-pull” operation is filtered with a low-pass filter 105, to generate a voltage Vo. The voltage Vo is used as a control signal for VCOs and VCDs (see Japanese Laid-Open Patent Publication No. 2000-82954 (page 6, FIG. 6), for example).
In control of switches in a charge pump circuit, switching noise occurs in a current flowing under the push-pull operation. This switching noise will be described with reference to FIG. 8.
A switch made of a pMOS transistor is taken as an example in this description. First, during the OFF period until the switch is turned ON, “feedthrough noise” occurs in which the charge in an output capacitance is released and moves toward the gate via a fringe capacitance and the like of the switch. Once the switch is turned ON, “injection noise” additionally occurs in which the charge in the output capacitance is released and moves toward the gate via a gate capacitance and the like of the switch (period shown by A in FIG. 8). Therefore, the output current is not immediately supplied to a load capacitance as the defined value Io, but is used for charging of the fringe capacitance and the gate capacitance for a while, and the remaining charge is supplied to the load capacitance (period shown by B in FIG. 8). If the current is not from an ideal current source, the output voltage value of the current source changes under the influence of ON resistance of the switch with the passage of time, and thus the current value continues failing to reach the defined value Io (period shown by C in FIG. 8).
Upon turning OFF of the switch, the charges stored in the fringe capacitance, the gate capacitance and the like are output at one time (feedthrough noise and injection noise), and as a result the output current exceeds the defined value Io for a short time (period shown by D in FIG. 8). The feedthrough noise and the injection noise are then converged, so that the value of the output current is converged to zero (period shown by E in FIG. 8).
When a charge pump circuit is operated at high speed, a charge error caused by switching noise becomes too large to be neglected with respect to the charge moving under the push-pull operation. For example, assuming that the current source supplies a current of 5 μA and the charge pump circuit is operated at 250 MHz, the charge supplied with one time of switching is 5 f coulomb (C) (5 μA×1 ns). If the switch has a fringe capacitance of 1 fF, the charge caused by feedthrough noise occurring when the switch is operated at 4V is 4 fC (=4V×1 fF). In this case, therefore, the charge error caused by the noise is roughly the same in magnitude as the charge supplied from the current source. In other words, the charge moving under the push-pull operation of the charge pump circuit includes a considerable amount of noise-caused error.
In relation to the above, the following problem arises. That is, the noise-caused charge error differs among transistors different in attribute. In particular, n-channel transistors and p-channel transistors differ from each other in the relationship between the control voltages with which turning ON and OFF are determined and the threshold voltage of each transistor, and thus greatly differ from each other in the charge amount caused by charge injection noise. Considering the characteristics of transistors, it is virtually impossible to suppress occurrence of feedthrough noise and injection noise. Moreover, injection noise changes with variations of the power supply and the process. It is therefore extremely difficult to have an equal amount of switching noise for a p-channel transistor and an n-channel transistor. Therefore, in the conventional charge pump circuits, the switching noise is asymmetric due to the difference in attribute between the transistors, and thus it is virtually impossible to secure equilibrium between the charge formed with the push operation and the charge released with the pull operation.